Add test that makes sure text consts are in sync

src/__tests__/Layout-consts-test.js

@@ -0,0 +1,14 @@
+/* globals layoutTestUtils */
+
+var textSizes = layoutTestUtils.textSizes;
+var preDefinedTextSizes = layoutTestUtils.preDefinedTextSizes;
+
+describe('Layout tests consts', function() {
+  it('keeps browser text measurements in sync with predefined consts', function() {
+    expect(preDefinedTextSizes).toEqual(
+      textSizes,
+      'Looks like browser has updated its text measurements functions. ' +
+      'You need to update `preDefinedTextSizes` in Layout-test-utils.js'
+    );
+  });
+});